The present invention is a process for separating particulate silicon from a liquid or liquid by-product stream containing silanes. The described process comprises atomizing a liquid by-product stream containing a slurry of silanes and particulate silicon into a heated zone to effect vaporization of the liquid silanes, thus drying the particulate silicon. The particulate silicon is separated from the gaseous silanes by filtration or other suitable means. The separated solid and gaseous phases may be used as feed to the process generating the by-product stream or as a feed for other processes.
The present process is particularly useful for the recovery of silanes from the by-product stream resulting from the chemical vapor deposition (CVD) of silanes to form elemental silicon.
A typical by-product stream of a CVD process for forming elemental silicon from silanes contains silanes, disilanes, and finely dispersed particulate silicon. A common method for treating this by-product stream is incineration. Incineration methods typically require that the by-product stream be dilute in order to avoid hazardous concentrations of pyrophoric disilanes. This requirement results in large volumes, which makes the incineration process costly. The present process not only avoids the cost of incineration, but allows the silanes and particulate silicon to be recovered and recycled to the CVD process or used as feed materials for other processes.
Prior patents for treatment of by-products from a chemical vapor deposition process disclose treatment of a one-phase by-product comprising either a gaseous mixture or particulate silicon.
Masaru, U.S. Pat. No. 4,642,208, issued Feb. 10, 1987; and Masaru, U.S. Pat. No. 4,881,952, issued Nov. 21, 1989, disclose a method of capturing particulate silicon produced during a CVD process for forming amorphous silica films. The particulate silicon is collected by a vacuum suction means and mixed with a heat distillable liquid having an affinity for silicon, to form a slurry. The slurry is heat-distilled to form pellets or a solid mass.
Griesshammer et al., U.S. Pat. No. 4,515,762, issued May 7, 1985, describes a method of processing a by-product gaseous mixture from a silicon deposition process. The by-product gases comprise trichlorosilane, tetrachlorosilane, hydrogen, and hydrogen chloride. Chlorosilanes are recovered from the gaseous mixture by condensation. The remaining gaseous mixture is combusted in the presence of oxygen and a chloride to form oxide particles and recoverable hydrogen chloride gas.
Candlin et al., U.S. Pat. No. 4,666,990, issued May 19, 1987; and Candlin et al., U.S. Pat. No. 4,629,771, issued Dec. 16, 1986, describe a process for the preparation of a catalyst impregenated with one or more transition metal compounds. This process comprises forming a suspension from an inert liquid medium and at least one solid material, the suspension containing a compound of a transition metal which is either dissolved in the inert liquid medium or is present as a solid material suspended in the inert medium. Spray drying is used as a method of evaporating the inert liquid medium for the purposes of (1) separating the particulate material from the inert liquid medium; and (2) allowing the dissolved transition metal to coat the particulate material and forming the transition metal catalyst.
The described art does not teach the instant process where a by-product, containing particulate silicon suspended in liquid silanes, is separated by atomizing the by-product into a heated zone which causes vaporization of the liquid silanes, thus facilitating separation of the silanes from the particulate silicon.